The invention relates to a control method for a biaxial wheel test stand for the simulation of driving loads which consists of a load unit for adjusting the vertical and horizontal forces and a pivot head for adjusting the camber angle of the wheel to be tested. The test stand further includes a drive unit with a driven drum having starting rings to which the wheel to be tested is pressed by the load unit. The simulation is controlled by adjusting the vertical and horizontal forces along with the change in camber angle.
A vehicle wheel is subject to extreme and continually changing loads during a real driving environment. The vehicle wheel, as a safety unit on a vehicle, must be able to withstand these loads during its entire time of use. When a new wheel is developed, the form of the wheel, the material thickness and the type of material have to be chosen in such a way that a sufficient operating strength is obtained with minimum weight. For testing the operating strength, road tests are carried out on vehicles, e.g. on suitable test tracks on the one hand, on the other hand, different test methods are used, in order to simulate the driving loads on: the vehicle wheel. At the moment, a set of test methods are available, with which the static operating loads and the dynamic load components, which are to correspond as exactly as possible to a momentary driving condition, can be simulated. For the quality testing in the series production, test methods are usually used, which are carried out with a stationary, that is, a constant load. During the real driving operation, the radial and axial forces acting on the vehicle wheel are not constant, but depend on a plurality of factors. So as to create non-stationary radial and axial forces on a wheel to be tested, which can be changed with time, biaxial wheel test stands have been developed. A corresponding biaxial wheel test stand (ZWARP) has been used by most wheel manufacturers since about 1989, and is for example described in its structure in xe2x80x9cAutomobiltechnische Zeitschriftxe2x80x9d, 88 (1986), page 543 pp. The wheel to be tested rotates with the mounted tire on the inside of a drum with starting rings, which is driven by a drive unit, and is pressed against the drum by means of a load unit. The load unit of the wheel test stand (ZWARP) is applied by two separate servo-hydraulic load cylinders, which are arranged perpendicular to one another on horizontal carriages with double columns. One of the load cylinders is a vertical load cylinder for adjusting a vertical force, the other is a horizontal load cylinder for adjusting a horizontal force. So as to be able to achieve an approximation to real wheel loads, the camber angle of the wheel can be adjusted relative to the drum by means of a camber cylinder secured to a pivot head.
The biaxial wheel test stand (ZWARP) has proved itself during use. However, the simulations on the wheel test stand only lead to useful results if the access parameters for the wheel test stand (ZWARP) get as close as possible to the stress condition during the real driving operation. So as to fulfill this condition, the stresses of a test wheel depending on the corresponding wheel geometry were measured up to now in a real road test with intensive measurements using strain gauges (DMS). For adjusting the access parameters for the biaxial wheel test stand (ZWARP), the individual access parameters (vertical force, horizontal force, camber angle) are varied in an iterative process, until the previously obtained strain and tension variations at characteristic wheel parts during the real road test are also measured at the same wheel parts during the simulation test. The adjustment of the horizontal and the vertical load cylinders takes place by controlling the force, the adjustment of the camber angle takes place by controlling the angle. As the reference signal of the known control method for the biaxial wheel test stand is formed by the strain variations established during the road test, one cannot neglect the previous determination of the strain variations in the wheel parts by means of DMS measurements.
It is the object of the present invention to suggest a control method and a biaxial wheel test stand suitable for this, which enable an adjustment of the access parameters of the wheel test stand without previous strain gauge measurements.
This object is solved in its aspect according to the method of the invention, in that the adjustment of the horizontal force, the vertical force and the camber angle takes place in dependence on the wheel radial or restoring force and the wheel side force established during the real driving operation, and that the position of the point of application of the resulting force of the said wheel radial or restoring force and the said wheel side force is used as the control magnitude for the camber angle.
The wheel radial force and the wheel side: force can be measured in a simple manner during the road test with special measuring hubs which are independent of the wheel geometry and represent wheel-specific magnitudes which are dependent on the rim size, the tire, the vehicle and the test track. It has now been shown with tests at vehicle wheels, that the stress of a vehicle wheel in the wheel test stand is identical to the stresses of the vehicle wheel during the real road test, when the force resulting from the wheel radial force and the wheel side force when the tire contacts the road is identical or corresponds to a large extent to the force resulting in the wheel test stand (ZWARP) with regard to the amount, direction and position. After this hypothesis has been verified, it has been found that the position of the point of application of the resulting force of the wheel radial force and the wheel side force can be used as the control magnitude for the camber angle. As the previous time and cost consuming measuring series with strain gauge at the vehicle wheel and also during the simulation in the wheel test stand a measurement with a strain gauge can be foregone with the method according to the invention, the control method according to the invention for establishing the access parameters for the wheel test stand offers substantial time and cost advantages. It is an additional advantage that the influence of the tire and the tire air pressure in the ZWARP is considered or eliminated, as the data of the tire determined during the real road test are readjusted at the wheel test stand with the control method according to the invention.
With the preferred embodiment of the invention, the camber cylinder force is measured so as to enable the use of the position of the point of application of the force as a control magnitude. This measurement can take place in a particularly simple manner with a capsule-type dynamometer arranged at the camber cylinder. This procedure; has the advantage that the values measured at the camber cylinder are not falsified by friction losses or measurement errors, as could for example occur during the pressure measurement at the camber cylinder.
So as to be able to carry out the control method with well-structured algorithms, the position of the point of application of the resulting force is defined by the distance of the point of application of the force from the center of the wheel with a preferred embodiment of the method. With this arrangement of the method, the algorithm for the position of the point of application of the force, the equation dependent on the data of the tire which can be calculated and the geometric relationship in the wheel test stand can be determined as
RDS=(MFs+Faxc3x97Rdyn)/Frxe2x88x92a1 
wherein
MFs: moment of the camber cylinder force around the head pivot point S;
Fa: axial wheel side force according to the road test;
Fr: radial wheel restoring force according to the road test;
Rdyn: dynamic roll radius; and
a1: distance between pivot point S of the camber angle and the center of the tire.
With the preferred embodiment of the method, the vertical force, the horizontal force and the camber angle are changed by means of a control and an evaluation unit until an unambiguous solution is found for the above algorithm, together with the algorithms
Fr=xe2x88x92Fhxc3x97sin(y)xe2x88x92Fvxc3x97cos(y); and 
Fa=xe2x88x92Fhxc3x97cos(y)+Fvxc3x97sin(y) 
or 
Fv=xe2x88x92Frxc3x97cos(y)+Faxc3x97sin(y); and 
Fh=xe2x88x92Frxc3x97sin(y)xe2x88x92Faxc3x97cos(y). 
with given Rdyn, RDS, Fa and Fr.
With a further preferred embodiment of the method, the position of the point of application of the force is moved into the tire center in a first approximation, that is, the center offset from the point of application of the force of the wheel center is set to zero. In extensive measurements it was surprisingly established, that, with this approximation solution, if the wheel radial force and the wheel side force are for example known from the road test, but not the position of the point of application of the force, a sufficiently exact correspondence of the access parameters adjusted at the wheel test stand with the vehicle loads resulting from the real driving operation can be produced.
A particularly suitable wheel test stand for carrying out the method is characterized in that the wheel radial force and the wheel side force known from the real driving operation can be entered into the control and evaluation unit as input magnitudes and that a measuring device is provided which measures the camber cylinder force acting on the camber cylinder. As already explained above, with the preferred embodiment of the wheel test stand, the measuring device consists of a capsule-type dynamometer assigned to the camber cylinder, as the capsule-type dynamometer enables a very simple and exact measurement of the camber cylinder force free of friction losses and hysteresis errors.